1. Technical Field
Exemplary aspects of the present disclosure generally relate to a detachably attachable process unit including, as a single integrated unit, a latent image bearing member, a charging device that charges a surface of the latent image bearing member, and a development device, and an image forming apparatus including the process unit.
2. Description of the Related Art
In known electrophotographic image forming apparatuses, a charging device uniformly charges a surface of a photosensitive member as a latent image bearing member, and an optical writer projects a light beam onto the charged surface of the photosensitive member to form an electrostatic latent image on the photosensitive member. A development device develops the electrostatic latent image formed on the photosensitive member with toner to render the electrostatic latent image visible as a toner image. Subsequently, the toner image is directly transferred from the photosensitive member onto a recording medium or is indirectly transferred from the photosensitive member onto a recording medium via an intermediate transfer member.
The charging device of the image forming apparatuses of this kind includes a charging roller to which a charging bias is applied. Electrical discharge occurs between the charging roller and the photosensitive member, thereby charging uniformly the surface of the photosensitive member. The charging device, the photosensitive member, and the development device are held as an integrated single unit by a common holder and are detachably attachable as a single integrated unit relative to a main body of the image forming apparatus.
When a charge potential of the photosensitive member deviates significantly from a target value, a background potential (i.e., a potential difference between a potential of a non-image formation area of the photosensitive member and a surface potential of a development member (i.e., a development roller) of the development device) development bias and a dark portion potential) becomes too much or too little, causing imaging failure. When the background potential is insufficient, the toner on the development member of the development device moves undesirably to the non-image formation area or a background portion of the photosensitive surface, resulting in undesirable toner adhesion onto a recording medium. This contamination of the non-image formation area of the surface of the photosensitive member is hereinafter referred to as background fogging.
In a case in which the image forming apparatus employs a two-component development method using a two-component developer including toner and magnetic carrier, with an excess background potential, the magnetic carrier on the development member moves to the surface of the photosensitive member undesirably. This is known as carrier adhesion.
In view of the above, it is necessary to control the charge potential of the photosensitive member to attain the target value. However, in the image forming apparatus using a plurality of process units for different colors, the charging bias to attain the target charge potential for the photosensitive member varies between the process units. More specifically, the thickness of the surface layer of the photosensitive member varies between the photosensitive members in the process units. Furthermore, the electrical resistance of the charging roller also varies between the charging rollers in the process units. Due to these variations, a discharge start voltage between the charging roller and the photosensitive member also varies between the process units. As a result, the charging bias to charge the surface of the photosensitive member to a target voltage, for example, −700V, varies between the process units.
If all the process units employ the same charging bias, the charge potential of the photosensitive member may deviate significantly from the target value depending on the process unit, resulting in the background fogging and carrier adhesion onto the photosensitive member. In particular, in order to realize cost savings, the charging bias may consist only of a direct-current component. In such a case, the discharge start voltage between the charging roller and the photosensitive member is affected significantly by the charge potential of the photosensitive member. The variations in the discharge start voltage between the process units easily cause the background fogging and carrier adhesion to the photosensitive member.
In view of the above, there is demand for an image forming apparatus that is capable of preventing the background fogging and carrier adhesion derived from variations in the discharge start voltage between the process units.